The retroreflective security films disclosed in U.S. Pat. No. 3,801,183 (Sevelin) provide many features making them well-suited for use as security overlays which can be laminated over a document or portions thereof. Such films are commonly used on such documents as identification cards, driver licenses, or documents of title, providing means for authenticating the documents as well as means for readily detecting tampering or attempted tampering therewith. Such security films are typically made with reflective layers which are substantially transparent in diffuse light conditions, thereby permitting the information on the document surface to be easily read. Commonly, the entire surface of the film has some level of retroreflection that is readily observable with a retroreflective viewing device such as disclosed in U.S. Pat. Nos. 3,767,291 (Johnson) and 3,832,038 (Johnson). Typically, any penetration of the film is disruptive to retroreflection by the film and is thus readily apparent under retroreflective viewing conditions, thereby providing means for enabling detection of tampering. Legends or patterns of enhanced retroreflection can be provided as desired in the form of logos, special messages, or other images to provide means for authentication.
U.S. Pat. No. 4,099,838 (Cook et al.) discloses a retroreflective sheet material which carries retroreflective legends provided by a dielectric mirror wherein the mirror is patterned so that the reflectivity from the mirror carries a pattern of contrasting colors. The patent discloses that security films of the type disclosed in U.S. Pat. 3,801,183 may be made with such patterned reflectivity.
Previously known security films have been provided both in "exposed-lens" (i.e., with the front surfaces of the microspheres protruding from the front surfaces of the film) and "embedded-lens" (i.e., with the front surfaces of the microspheres being embedded within the film) embodiments.
However, previously known security films may exhibit certain disadvantages, including limited resistance to abrasion, tendency to deform or for retroreflectivity to be disturbed when subjected to moderate pressures, limited durability under certain conditions of use such as are encountered by employee identification cards and driver licenses, and particularly in the case of exposed-lens constructions, tendency to accumulate dirt between microspheres and tendency for microspheres to dislodge when the sheeting is exposed to high humidity.